1. Field of the Invention
The present invention relates to a machine tool provided with a clamping jig for a rotationally-symmetrical body, and particularly to a machine tool provided with a jig which clamps a workpiece when the workpiece of a rotationally-symmetrical body such as an impeller used in a compressor or the like is subjected to a cutting work.
2. Description of the Related Art
Recently, an impeller used in a vehicle, an air conditioner, or the like is increasingly formed by a cutting work. In the cutting work of the impeller, particularly in a rough machining stage before a finishing machining, a cutter having a large diameter is used or the cutting speed is increased in order to efficiently perform the cutting work and reduce the machining time, in many cases. In addition, in order to use the cutter having a large diameter or increase the cutting speed, as a method of bearing a high cutting load, an impeller workpiece needs to be fixed to a cutting jig. In addition, for good rotational balance of a product after being subjected to the cutting work, the cutting work needs to be performed without eccentricity of a workpiece.
JP 2011-513077 W discloses a machine tool for cutting a workpiece having a center hole, which includes a workpiece support table on which the workpiece is placed, a workpiece holding portion having a gripping arm which presses the workpiece from above, and a driving unit which rotates the workpiece around the center axis line thereof, wherein the workpiece support table has a protrusion which is fitted into the center hole of the workpiece from below, a tip end of the gripping arm has a protrusion which is fitted into the center hole of the workpiece from above, and the protrusion presses the workpiece downward.
However, in the apparatus disclosed in JP 2011-513077 W, a mechanism for bringing the protrusions into contact with the hole is provided, and the angle of the end surface of the hole and the inclined surface of the protrusion have line contact therebetween. Therefore, there is concern that non-contact may occur depending on the precision of the shapes of the hole and the protrusions.
JP 2005-538854 W discloses an apparatus including a tie rod and a support element to clamp a rotationally-symmetrical body for machining, in which a spring force in a direction opposite to a tensile force is applied to the support element, the support element is configured to be bent in an axial direction when the rotationally-symmetrical body abuts the support element because the spring force is weaker than the tensile force, and the support element is elastically supported by a stop of the apparatus and is allowed to move in an axial direction of the clamped rotationally-symmetrical body.
However, in the apparatus disclosed in JP 2005-538854 W, a mechanism for bringing the support element having a centering action into contact with the workpiece is used, and the support element is movable in the axial direction.
In the technique disclosed in any of JP 2011-513077 W and JP 2005-538854 W described above, when a workpiece is fixed to a jig, a mechanism for centering the workpiece on the jig is not present, and thus the workpiece is not centered and is clamped with eccentricity. Therefore, when a cutting work is performed thereon, there is concern that the rotational balance of a product after the cutting work may be degraded.